A CDMA (Code Division Multiple Access) system is a self-interfering system, wherein a transmit power of a mobile station is an interference to other users occupying the same frequency in the cell, and a Near-Far Effect is a predominant problem. The so called Near-Far Effect refers to that, when mobile phones with different distances from the base station are used simultaneously, a signal with a higher power and nearer to the base station will suppress a signal with a lower power and farther from the base station. Therefore, it is expected to make the transmit power of each mobile station maintain a minimum level to meet the requirement for communication as far as possible, and to make the mobile station obtain a basically same power on the base station side regardless of the distance from the base station and the channel condition. A power control technology is a technology to overcome the Near-Far Effect for the CDMA system. It may decrease an average transmit power and reduce an inter-system and an intra-system interference.
The power control may be divided into an open loop type and a closed-loop type, wherein the closed-loop type power control may be further divided into an outer-loop power control and an inner-loop power control. Usually, the outer-loop sets target values (such as a received power target value and a signal-to-noise ratio target value, etc.) according to the quality of the service such as BER (Bit Error Rate) and BLER (Block Error Rate). The inner-loop adjusts the transmit power according to a comparison result between the signal quality and the target value set by the outer-loop.
According to directions of the link, the power control may also be divided into an uplink (backward) power control and a downlink (forward) power control.
In a conventional downlink power control algorithm, only the quality of signal received by the terminal is considered, while the power difference in signals is not considered. Taking the inner-loop downlink power control as an example, the inner-loop downlink power control is accomplished by the base station with the assistance of a terminal. The terminal measures the signal quality (such as the received power or the signal-to-noise ratio) and compares the signal quality with the target value. If the signal quality is greater than the target value, a command for decreasing the transmit power is generated; otherwise, a command for increasing the transmit power is generated. If the transmit power needs not to be changed, in other words, if it is allowed that the transmit power is not adjusted when the signal quality is in a certain range around the target value, then in the above determination process, it is determined that the command for decreasing the transmit power is generated when the signal quality is greater than the target value by a certain threshold and the command for increasing the transmit power is generated when the signal quality is less than the target value by a certain threshold; otherwise, a command for not changing the transmit power is generated. Then, the control command is sent to the base station via a control channel or in other ways, and the base station performs a corresponding adjustment in a designated or predefined step size according to the received control command, so that the base station may keep an ideal transmit power.
In a practical wireless communication system, in the direction of the downlink, because the distances from different terminals to the base station are different, the required transmit powers meeting the quality of the service are also different. Usually, the transmit power of the base station needed by the terminal farther from the base station is higher, and the transmit power of the base station needed by the terminal nearer to the base station is lower. If the base station does not employ the downlink power control technology, the transmit power of the base station for each terminal is the same, and no Near-Far Effect exists. However, the transmit power of the base station must be configured according to the coverage area or the terminal farthest from the base station, thus great power will be wasted, the inter-system interference will be increased, and the system capacity will be limited. If the downlink power control technology is employed, the transmit power of the base station for each terminal will be different, thus the Near-Far Effect may occur. If the difference between the transmit powers is too great, a signal with low power may be submerged in a signal with high power at any moment, and the stability of the wireless link will be influenced, thus the quality of communication may not be guaranteed. However, the power difference between signals may also be resolved or overcome in other technical solutions, such as the joint detection technology. Theoretically, ideal joint detection may completely overcome the Near-Far Effect. However, it is difficult to be implemented in practice. In the prior system, it usually employs a suboptimal joint detection algorithm or a joint detection algorithm simplified according to a certain model hypothesis, thus, the ability to overcome Near-Far Effect is limited. Too large power difference will degrade the demodulation performance.
Thus it can be seen that in the downlink power control algorithm, the terminal does not consider the power difference between different signals, therefore, it is adverse to guarantee the communication quality of the system.
Optionally, the limitation on the transmit power difference between all the code channels of the same carrier frequency and the same time slot (for a time-division system) may also be implemented in the base station. But in the existing protocol, the power difference between code channels is not limited; in other words, the base stations of different manufacturers may not consider the difference between all the code channels of the same carrier frequency and the same time slot (for a time-division system). Therefore, the power for receiving signals of the terminal belonging to the base station and being nearer to the base station may be much lower than that of other terminal or terminals. As a result, the demodulation performance of the terminal may be degraded and the wireless link may be unstable and easy to be broken and so on. Therefore, the quality of the communication cannot be guaranteed.